Apparatus for carrying out accelerated tests for light fastness and weather resistance of materials is known. One such apparatus will be described with reference to FIG. 1. In FIG. 1 there is illustrated a sectional elevation view of a conventional accelerated light fastness/weather resistance testing machine having a test tank 1 which includes a light source 2 at the center thereof. A sample 4 is mounted on a sample rotating frame 3 rotatable about the light source 2 with the sample facing the light source, the sample 4 deteriorating during a test period in which it receives radiant heat emitted from the light source 2.
A temperature sensor 5 is fixed in the test tank 1 for measuring the dry-bulb temperature in the interior of the test tank 1, and a black panel thermometer 6 is mounted on the sample rotating frame 3 in juxtaposed relation to the sample 4, the indicated temperature being visually readable.
The temperature sensor 5 is used for controlling an air temperature regulator (not shown) and is located in an air outlet, the temperature sensor 5 serving to set the air temperature regulator so that the black panel temperature is held at a constant value.
Alternatively, in place of the air temperature regulator, there has been used a means for mounting a black panel temperature regulator 7 on the sample rotating frame. This is called a black panel temperature control system.
The temperature of the sample surface is raised due to the increase of the air temperature in the tank and radiant heat emitted from the light source 2, and a control signal is taken out from the temperature sensor 5 for the air temperature regulator or the black panel temperature regulator 7 to make temperature control possible, so that the temperature of the sample surface can be held constant.
The black panel thermometer 6 comprises a bimetal type dial thermometer, the barrel of which is mounted on a stainless plate, with black paint applied thereon.
An air flow regulator 8 is provided on the side of the test tank 1 and a blower 9 is provided at the base of the testing machine, the air regulator 8 and the blower 9 being interconnected by a circulation duct 10. A motor associated with the air regulator 8 is operated from the signal transmitted from the temperature sensor 5 through the air temperature regulator or the black panel temperature regulator 7 to thereby open and close a damper 11.
When the temperature in the tank becomes higher than the set temperature of the air temperature regulator or the black panel temperature regulator 7, the damper 11 is brought into a fully open state (FIG. 1 position) so that fresh air is introduced into the test tank 1 by means of the blower 9 through the air flow regulator 8 and the duct 10 and is then discharged through an exhaust duct 12.
When the tank temperature becomes lower than the set temperature, the damper 8 is fully closed (about 90.degree. clockwise from the FIG. 1 position) so that no fresh air is introduced, but instead the air in the machine is caused to circulate within the tank through the circulation duct 10.
In this manner, according to the prior art apparatus, the tank temperature is regulated by switching the air flow path by switching the damper 8 between a fully open state and a fully closed state.
Such prior art testing apparatus is shown in Japanese Industrial Standard B7751-1974, "Ultraviolet Ray Carbon Arc Lamp Type Light Resistance Testing Machine", Japanese Published Pat. No. 28956/1969, and U.S. Pat. No. 4,025,440.
However, for completely testing the light fastness/weather resistance by an accelerated test, the testing apparatus must maintain the temperature of the sample surface at a predetermined temperature, and must be able to direct light radiated from an artificial light source onto the sample and to simulate rainfall thereon, and to evaluate the extent of aging and deterioration of the sample by measuring changes in the sample over a period of time.
In such a light fastness/weather resistance testing machine, the most important point among the testing conditions is to hold the sample temperature uniform. To this end, it is necessary to simultaneously regulate both the air temperature in the testing tank and the black panel temperature so that they remain at a constant level irrespective of variations in the ambient temperature at the test site.
In the conventional machine as described above and in other similar machines, when the temperature of the sample surface is increased, the air flow regulator is operated to fully open the damper so that cool outside air is introduced to replace hot air in the tank, thereby cooling the interior of the tank. When the temperature in the tank is too low, the air flow regulator is operated to fully close the damper so that air from the tank is caused to recirculate thereinto.
In such a case, assuming that the air temperature regulator is preset to 40.degree. C., for instance, the black panel thermometer 6 will display 63.degree. C. This is for the condition that the ambient temperature ranges from 20.degree. C. through 25.degree. C.
However, whenever the ambient temperature varies outside the above-described range, the preset temperature must be modified accordingly. This is not only very inconvenient but often impossible in a continuous test over a long period of time, with the result that precise test results can not be obtained.
To eliminate the foregoing disadvantage, there has been used a means for using the black panel temperature regulator 7 to set the temperature of the black panel temperature regulator 7 to 63.degree. C. in advance. However, this means has also created a problem as follows.
Because the temperature sensor of the black panel temperature regulator 7 which is a black stainless plate is relatively insensitive to variations in ambient temperature, it can not quickly detect such variations even if fresh air is introduced into the testing tank. As a result, the black panel temperature regulator 7 does not respond quickly, so that it gives an inaccurate reading, thereby making it impossible to indicate the precise temperature.
At the same time, the black panel thermometer 6 also suffers from a similar problem.
Further, when the ambient temperature is low, at the time of introduction of outside air, the sample is subjected to a sudden thermal stress because very cool air suddenly strikes the sample surface.
Consequently, the test results differ from those obtained by testing in open air at ambient temperatures.
Moreover, the very cool air strikes the sample surface beginning from its lower portion, which gives rise to another problem, namely the generation of a temperature difference between the upper and lower portions of the sample surface. Accordingly, this causes a difference in test results between the upper and lower portions of the sample, thereby preventing a precise evaluation of the characteristics of the sample.
When using a black panel temperature control system, for instance, assuming the ambient temperature to be 10.degree. C. and a setting temperature of 63.degree. C., the air temperature in the testing tank is about 40.degree. C. As the air flow regulator is operated, air having a temperature 30.degree. C. lower than the tank temperature strikes the sample surface, thereby causing thermal stress. Also, the temperature of the sample surface is lowered to 10.degree. C. in the lower portion, 15.degree. C. in the middle portion and 25.degree. C. in the upper portion, so that precise test results can not be obtained.
Alternatively, when using the air flow regulator 8 only, even if the temperature regulator is set to hold the temperature of the sample surface constant, i.e., the temperature indicated by the black panel thermometer 6, the air temperature in the test tank can not be held constant for causing the black panel thermometer to display a constant temperature, because the temperature in the test tank varies largely depending on the ambient temperature of air introduced into the tank for temperature control.
When attempting to hold the black panel temperature constant at 63.degree. C., for instance, the temperature in the test tank has to be set relatively high, i.e. at 43.degree. C., for an outside air temperature of 5.degree. C., but has to be set relatively low, i.e. at 38.degree. C., for an outside air temperature of 30.degree. C. It is in this way that the air temperature in the tank varies depending on the outside air temperature.
Further, the differences in temperature at the upper, middle and lower portions of the sample surface cause differences in the rate of deterioration of the sample among the upper, middle and lower portions thereof, thereby preventing precise evaluation of the test results.
Moreover, the Japanese Industrial Standard allows the black panel thermometer to be 63.degree. C..+-.3.degree. C. This is because, in the normal situations, the testing machine will not operate sufficiently accurately for an allowance of less than .+-.3.degree. C.
Still further, as previously noted, the black panel thermometer is incapable of quickly responding to variations in tank temperature. This causes a lag between the temperature displayed by the thermometer and the actual temperature of the sample, so that precise test results can not be obtained.
To solve these problems, it has been proposed to install the testing machine in an environment of small temperature variation, for instance an air-conditioned space. However, this requires very expensive equipment.
As a practical matter, such a testing machine will normally be installed in a space which is not air-conditioned in many cases. Thus, the temperature in the installation site of the testing machine varies with the time of day and the season, as shown in FIGS. 7 and 8, respectively. In some regions, the difference in temperature between the summer and winter seasons is 45.degree. C.
From the above, it can be seen that it is practically impossible to hold the temperature constant at all test sites as well as impossible to prevent variations in ambient temperatures.